Conventional mechanical valve engines and electromechanically actuated valve engines may exhibit increased emissions and reduced transient response performance if air-fuel maldistribution exists between cylinders. Engine emissions may increase due to air-fuel mixtures that are too rich or too lean. For example, during a cold engine start it is desirable to operate cylinders at a lean air-fuel mixture to reduce hydrocarbon emissions. However, if a maldistribution of air-fuel exists between cylinders, one cylinder may produce a low hydrocarbon exhaust gas while another cylinder may misfire because of a leaner air-fuel mixture. Further, during acceleration from idle, large rates of change in cylinder air amount may result in calculation errors that can produce air-fuel errors. The air fuel errors may cause an engine to hesitate or stumble.
One method to adjust individual cylinder fuel amounts during engine operation is described in U.S. Pat. No. 5,548,514. This method estimates air-fuel ratios in individual cylinders of a multi-cylinder internal combustion engine from an output of an air-fuel ratio sensor. The method uses a linear air fuel sensor to extract individual cylinder air-fuel ratios from a mixed exhaust gas passage. A sensor state equation is developed from a first order lag approximation. Further, the method expresses air-fuel ratio at the confluence point as the sum of products of the past firing events, where individual cylinder histories are expressed as weighting factors.
The before-mentioned approach can have several disadvantages. Namely, the approach expresses air-fuel ratio as a sum of weighted cylinder histories, where the weighting factors are constant. However, gas flow properties within an exhaust manifold are subject to engine operating conditions, i.e., mass flow rate, temperature, pressure amplitude, and pressure frequency. For example, an exhaust manifold may have certain resonance frequencies that may be excited during different speed/load conditions of an engine. Using weighting factors may not be sufficient to express each cylinder contribution at the confluence point under these conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed various solutions, described further below. Further, they have recognized that it is desirable to reduce cylinder maldistribution by compensating cylinder air amounts or injected fuel amounts.
One embodiment includes a method to control air-fuel ratios in individual cylinders of an internal combustion engine with electromechanical valves. The method comprises: operating at least an electromechanical valve in each cylinder combusting an air-fuel mixture during a cycle of said engine; and correcting an air and fuel mixture supplied to each cylinder combusting an air-fuel mixture based on a desired air-fuel mixture in said each cylinder combusting an air-fuel mixture.
Another embodiment includes a method for adjusting air-fuel in an internal combustion engine with electromechanical valves. The method comprises: selecting a group of cylinders each cylinder of said cylinder group containing at least an electromechanical exhaust valve; adjusting exhaust valve timing based on a predetermined schedule; sampling at least one oxygen sensor located in an exhaust system downstream of said group of cylinders based on said adjusted exhaust valve timing; and adjusting individual fuel amounts injected into said cylinders of said cylinder group based on a difference between an actual air-fuel ratio, determined from said oxygen sensor, and a reference air-fuel ratio. These methods can be used to reduce at least some of the above-mentioned limitations of the prior art approaches.
For example, by selectively adjusting electromechanical valve events in an internal combustion engine and then adjusting fuel or air at the active cylinders, individual cylinder air-fuel can be improved.
Engines with multiple cylinders may exhibit maldistribution for a number of reasons including but not limited to: varying port geometry between cylinders, electromechanical valve variation, injector location variation, port surface finish differences, and engine temperature variation between cylinders. These individual cylinder air-fuel variations are difficult to differentiate between cylinders because of manifold mixing, oxygen sensor response, and varying engine operating conditions. The inventors herein have reduced these undesirable influences and improved individual cylinder air-fuel detection and control by, in one example, extending the separation between adjacent cylinder exhaust intervals by control of electromechanical cylinder valves. For example, an eight-cylinder engine operating all eight cylinders has a 90° crank angle interval between individual cylinder events. Depending on exhaust manifold volume, engine speed, engine load, and other factors, a considerable amount of exhaust gas mixing may occur before the exhaust gas reaches an exhaust gas sensor. As a result, determining the air-fuel ratio of a single cylinder may be difficult. By increasing the interval between cylinder events, (e.g., deactivating four cylinders in an eight cylinder can increase the crank angle interval between cylinder events to 180°), mixing of exhausted gases from individual cylinders may be reduced since cylinder pressures have additional time to generate flow toward the atmosphere. By reducing exhaust gas mixing, individual cylinder air-fuel ratio control may be improved because past histories of adjacent cylinder combustion events may contribute a smaller fraction to the observed air-fuel ratio.
Further, the invention herein may be utilized in fuel saving cylinder modes. In these modes, an even greater concentration of the monitored cylinder air-fuel ratio may be observed. For example, combustion events occur every 180 degrees for a 4-cylinder, 4-stroke engine. When the same engine is operated in 2-cylinder mode, at the same torque, the cylinder charge mass increases, improving air-fuel detection at the confluence point, i.e., the point where cylinder exhaust combine and are detected by the oxygen sensor. In this example, air-fuel detection is improve by signal separation, since the number of crank angle degrees increases between cylinder events, and by increasing the cylinder mass fraction in the exhaust gas of the cylinder of interest.
In this way, improved individual cylinder air-fuel detection can be achieved thereby reducing engine emissions and improving transient response. Further, improved fuel economy can be achieved by accurately matching fuel and air charge amounts.
The above advantages and other advantages and features will be readily apparent from the following detailed description of the embodiments when taken alone or in connection with the accompanying drawings.